Method and apparatus for energy recovery in an environmental control system

ABSTRACT

Control apparatus for an environmental control system comprises input circuitry receiving environmental information and output circuitry for controlling an HVAC system. Processing circuitry in the controller configures the output circuitry based at least in part on the signals received on the input circuitry. Information about the status of the HVAC system may be transferred to system administrators using a wireless link.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is a continuation of, and claims priority to U.S.patent application Ser. No. 09/351,974, filed on Jul. 12, 1999, now U.S.Pat. No. 6,176,436, which application in turn is a continuation of, andclaims priority to U.S. patent application Ser. No. 08/933,871, filed onSep. 19, 1997, U.S. Pat. No. 6,062,482. The content of U.S. patentapplication Ser. Nos. 09/351,974 and 08/933,871 are hereby incorporatedby reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to controllers for heating, ventilation,and air conditioning (HVAC) systems. More specifically, the presentinvention relates to dynamic, digitally implemented HVAC control.

2. Related Art

Efforts to manage the environmental condition of a room, building, orother controlled space have resulted in a wide variety of systems forcontrolling the operation of heaters, air-conditioning compressors,fans, and other components of HVAC equipment. The simplest and most wellknown form of such control is simply a thermostat which senses thetemperature of a controlled space, and sends signals to the HVAC systemif the temperature is above or below a particular setpoint. Upon receiptof these signals, the HVAC system supplies cooled or heated air to thespace as called for by the thermostat.

Although this simple system is adequate in many instances, improvementshave been and are desired. Many aspects of the development of HVACcontrol apparatus and algorithrns focus on increasing occupant comfortby controlling the environmental condition more tightly. A competingconcern, however, is minimizing the energy consumed by the HVAC system.It can be appreciated that the various control schemes utilized impactthe energy consumption of the HVAC system. In the past, efforts toaddress excessive energy consumption have focused on determining when aspace is unoccupied or otherwise has a lower requirement forenvironmental control. Examples of these systems include those describedin U.S. Pat. No. 4,215,408 to Games, et al., and U.S. Pat. No. 5,395,042to Riley, et al. In U.S. Pat. No. 557,317 to Harmon, Jr., an HVACcontroller includes a drifting “dead-band”, so that energy consumptionis reduced due to the allowance of wider swings in the temperature ofthe controlled space. In the Harmon, Jr. system, occupant comfort issaid to be maintained because the rate of change of the temperature ofthe controlled space remains low.

One potential source of energy savings has thus far not been fullyexploited. This is the minimization of energy loss via heat conductionand radiation through exposed ducting and other components of the HVACsystem. This energy loss is exacerbated by the fact that a correctlysized HVAC unit will operate at fall rapacity only on the hottest orcoldest days of the year. The majority of the time, the unit is heatingor cooling the supply air to an average temperature which is hotter orcolder than that required to meet the demand for environmental controland maintain comfort for the occupants of the controlled space. Thisover capacity results in increased heat transfer from the system throughducting and other mechanical components of the HVAC system. Attempts torecover this escaping energy have thus far been limited. One systemattempts to recover escaping energy by extending the operating period ofthe supply air fan beyond that of the furnace or air conditioner.Another system establishes a fixed duty cycle for the furnace or airconditioner by measuring the temperature of the air being supplied tothe controlled space.

Although these systems do decrease energy waste somewhat, operatorcomfort is sacrificed to a degree which can be unacceptable. For onething, existing systems are not responsive to changes in externalconditions which cause changes in the energy needs of the controlledspace. Thus, a fixed duty cycle will not be appropriate for optimallysatisfying all calls for heating or cooling. In these cases, thecontrolled space may require an unacceptably long time to heat or coolto a given thermostat setpoint, leaving the occupants uncomfortable foran extended period. Furthermore, HVAC cycling during periods of highdemand for heating or cooling may cause noticeable fluctuations in thetemperature of the controlled space.

In addition to these factors, existing systems do not adequately providefor humidity control. It is recognized that humidity is a factor inoccupant comfort as well as temperature. Accordingly, systems whichalter HVAC system operation in response to humidity measurements havebeen produced. One example of such a system, adapted for controlling theair space inside an automobile, is described in U.S. Pat. No. 4,852,363to Kampf, et al. This system includes humidifiers and dehumidifierswhich are operated in response to a humidity measurement. Another morecomplex system, also adapted for control of an automotive HVAC system,is described in U.S. Pat. No. 5,579,994 to Davis, Jr. et al. In theDavis, Jr. device, several environmental parameters are sensed, and anoverall environmental control strategy is developed which is under fuzzylogic control.

Humidity control may also be performed by cycling an air conditioningunit, as the coils of the air conditioner remove water from the air inaddition to cooling it. As described in U.S. Pat. No. 5,346,129 to Shahet al., an air conditioning system can be run in response to relativehumidity measurements as well as temperature measurements made in thecontrolled space. Of course, this may cool the air more than is desiredby the occupants of the space, and accordingly, some systems willre-heat the dryer cooled air after it passes the condenser coils.

No presently available system, however, reduces HVAC energy consumptionwithout serious consequences to operator comfort resulting fromtemperature swings and higher humidity levels.

SUMMARY OF THE INVENTION

An HVAC control apparatus includes input circuitry configured to receiveinput signals from external sensors, processing circuitry coupled to theinput circuitry and configured to evaluate the signals, and outputcircuitry coupled to the processing circuitry. In this embodiment, theprocessing circuitry generates output signals which alter the state ofthe output circuitry. Input/output circuitry comprising a wirelesstransceiver is also coupled to the processing circuitry for transmittingdata to system administrators for diagnostic evaluation of said HVACsystem.

Methods of system management are also provided. In one environmentalcontrol system embodiment comprising (1) energy consuming heating and/orcooling components, (2) a digital heating and/or cooling componentcontroller, and (3) system administration facilities, a method of systemdiagnostics comprises transmitting information relating to the operationof the heating and/or cooling components from the digital controller tosystem administration facilities via a wireless communication link.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an environmental control system accordingto one embodiment of the present invention.

FIG. 2 is a flow chart illustrating the operation of one embodiment ofthe present invention.

FIG. 3 is a schematic/block diagram of an embodiment of an environmentalcontrol system incorporating aspects of the present invention.

FIG. 4 is a schematic/block diagram of one embodiment of an HVACcontroller according to some aspects of the present invention.

FIG. 5 is a composite of FIGS. 5A, 5B, SC, and 5D, and is a flowchartillustrating the operation of one embodiment of the present inventionduring a call for heating from a controlled space.

FIG. 6 is a composite of FIGS. 6A, 6B, 6C, and 6D, and is a flowchartillustrating the operation of one embodiment of the present inventionduring a call for cooling from a controlled space.

FIG. 7 is a composite of FIGS. 7A, 7B, and 7C, and is a flowchartillustrating another embodiment of the present invention during a callfor cooling from a controlled space.

FIG. 8 is a composite of FIGS. 8A, 8B, and 8C, and is a flowchartillustrating another embodiment of the present invention during a callfor heating from a controlled space.

FIG. 9 is a graph of gas use as a function of time for an HVAC systemwhich is operated in accordance with principles of the presentinvention.

FIG. 10 is a graph illustrating instantaneous and average energy outputto a controlled space when the HVAC system is operated in heating modein accordance with principles of the present invention.

FIG. 11 is a graph of input power as a function of time for an HVACsystem which is operated in accordance with principles of the presentinvention.

FIG. 12 is a graph illustrating instantaneous and average energy outputto a controlled space when the HVAC system is operated in cooling modein accordance with principles of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will now be describedwith reference to the accompanying Figures, wherein like numerals referto like elements throughout. The terminology used in the descriptionpresented herein is intended to be interpreted in its broadestreasonable manner, even though it is being utilized in conjunction witha detailed description of certain specific preferred embodiments of thepresent invention. This is further emphasized below with respect to someparticular terms used herein. Any terminology intended to be interpretedby the reader in any restricted manner will be overtly and specificallydefined as such in this specification.

Referring now to FIG. 1, an environmental control system according tosome aspects of the present invention is illustrated. A controlled space10 receives heated and/or cooled air from a heating, ventilation and airconditioning (HVAC) unit 20. The controlled space may be an automobileinterior, an office building, a barn or other animal enclosure, acomputer room, or any other space for which environmental control isadvantageous. Supply air may flow to the controlled space 10 via an airsupply duct 12. Return air from the controlled space 10 is routed backto the HVAC unit 20 via an air return duct 14. The HVAC unit 20typically comprises an oil or gas furnace for heating the air of thecontrolled space 10 as well as an air conditioning unit for cooling theair of the controlled space 10. The HVAC unit 20 may also comprise ventsand ducting (not shown) for drawing outside air into the system. TheHVAC system shown and described herein includes both air heating and aircooling apparatus, and is typical of many cannon installations. It willbe appreciated that the term “HVAC” as used herein also includes standalone heaters, stand alone air conditioners, heat pumps, and otherequipment that perform some or all of the environmental controlfunctions for a controlled space.

Coupled to the HVAC unit 20 is an HVAC controller 30. The HVACcontroller 30 may receive information regarding environmental conditionsand other information from the controlled space 10 via a sensor signalpath 22. The sensor signals may comprise electrical signals fromthermocouples, theorists, electronic humidity sensors, and other sensortypes well known to those in the art. The sensor signals may alsocomprise signals from the controlled space 10 calling for heating,cooling, or providing other information 30 about the condition and needsof the controlled space 10. Another signal path 24 is provided betweenthe HVAC controller 30 and the HVAC unit 20. This signal path preferablyincludes control signals from the HVAC controller 30, and may furtherinclude sensor signals which are routed back to the HVAC controller Itcan be appreciated that both signal paths are not always necessary todeliver the required information to the HVAC controller 30.

The HVAC controller 30 preferably operates to dynamically control theon/off state of components of the HVAC unit 20 to recover what would bewasted energy during those times when the HVAC unit 20 can meet thedemands of the controlled space without operating at maximum capacity.In some embodiments, information concerning the condition of thecontrolled space and information concerning the condition of the air theHVAC unit 20 is supplying to the controlled space 10 is received by theHVAC controller 30 via one or both signal paths 22 and 24. Thisinformation is used to determine whether or not the HVAC unit 20 canmeet the heating or cooling demands of the controlled space 10 at a lowenergy consumption rate while maintaining occupant comfort.

FIG. 2 is a flowchart illustrating one possible mode of operation for anHVAC system as shown in FIG. 1. In the first step 32, the HVACcontroller waits for a call for environmental modification from thecontrolled space 10. This signal may be sent by a thermostat inside thecontrolled space 10 to the HVAC controller 30 along signal path 22 ofFIG. 1. When a call is sensed, heat transfer begins to or from thecontrolled space at step 34. For example, when a call for heating isreceived, a gas burning furnace may be started, and heat energy will betransferred to the controlled space by ventilating the controlled spacewith heated air. When a call for cooling is received, heat energy istransferred from the controlled space to the outside environment by acooling coil system as is well known in air conditioning systems.

At step 36, the HVAC controller receives and evaluates informationregarding conditions in the HVAC system. As will be explained furtherbelow, these conditions may advantageously include various environmentaland physical conditions and parameters such as the approximatetemperature of the controlled space and the rate of change of thattemperature, the approximate temperature of the air being supplied tothe controlled space and the rate of change of that temperature, theapproximate humidity of the controlled space, the length of time theHVAC system has been in an on or off state, the length of time a callfor heating or cooling has been pending, etc. Based on the presentdisclosure, those of skill in the art will appreciate that not all ofthese parameters need to be evaluated to make or use the presentinvention. In addition, other parameters not specifically mentioned maybe used when, for example, certain parameters are especially relevant toa particular installation. The terms “physical” or “environmental”parameters or conditions are thus intended to include a wide variety ofinformation concerning the operation and status of the HVAC system andrelated devices and locations, and not simply the several described indetail herein with respect to certain specific embodiments of thepresent invention. As mentioned above, some or all of this informationmay be transferred from sensors to the HVAC controller 30 along signalpaths 22 and/or 24 of FIG. 1.

The next step 38 involves the determination of whether or not the rateof energy transfer to or from the controlled space can be reduced whilemeeting the call for environmental change within certain specifiedrequirements. In preferred embodiments, at least some of therequirements are designed to ensure that occupant comfort is notsacrificed to an unacceptable degree when reducing the rate of energytransfer. As will be explained below, however, occupant comfort is notthe only consideration at step 38. HVAC system operation requirementssuch as the prevention of over-cycling an air conditioning compressormay also be considered at step 38. In some advantageous embodiments ofthe present invention, the decision of step 38 is made based on theevaluation of physical and/or environmental parameters and conditionsperformed at step 36.

At step 40, if the energy transfer rate can be reduced within thespecified requirements, the system will initiate a reduction in theaverage heat energy transfer rate. In some embodiments, this step willinvolve shutting off some energy consuming portion of the HVAC system.For example, if the HVAC system is currently cooling the controlledspace, the system may turn off the air conditioning compressor and/oroutdoor fan. Furthermore, if the HVAC system is currently heating thecontrolled space, the system may close a valve which supplies gas to afurnace. Most preferably, the HVAC system continues to ventilate thecontrolled space, even though one or more energy consuming componentshave been turned off This continued ventilation is advantageous becausewhile the energy consuming component such as the compressor or outdoorfan is off, the continued ventilation allows heat transfer to continueby recovering energy from system components such as the ducting, othermechanical components of the HVAC system, and structural elements of thecontrolled space. Thus, energy consumption may be reduced, but usefulheat transfer may continue for a certain period of time.

If, on the other hand, the system determines that the energy transferrate cannot be reduced consistent with certain environmental and/oroperational requirements, at step 42 the system will transfer heatenergy at the maximum rate. In some embodiments, the decision totransfer heat energy at the maximum rate will be based on considerationssuch as a very low temperature in the controlled space when heating iscalled for, a very high temperature in the controlled space when coolingis being called for, or insufficient changes over time in the controlledspace temperature when the controlled space is calling for heating orcooling.

As illustrated by step 44 in FIG. 2, system operation also depends onwhether or not the call for environmental modification is still pending.If the call is no longer pending, at step 46 the HVAC system will shutdown, and the system waits for the next call for environmentalmodification back up at step 32. If the call is still pending, thephysical and environmental conditions of the system are again evaluatedat step 36, and a decision at step 38 is made regarding whether or notthe energy transfer rate can be reduced or should be set to the maximumrate.

It can be appreciated that at any given instance of executing step 38,the HVAC system may be in a state of reduced or maximum energy transferdepending on the results of any prior evaluations which have beenperformed, how long the system has been operating, and other factors.Whichever state the system is currently in, however, it is advantageousto conduct frequent reevaluations of the physical and environmentalparameters to determine whether or not the current operation mode isoptimal. Thus, dynamic control of energy consumption is produced. Thisin turn allows for increased energy efficiency while maintaining anacceptable level of comfort for occupants of the controlled space.

It will also be appreciated by those of skill in the art that in manyembodiments, the steps illustrated in FIG. 2 may be performed in variousorders other than that explicitly shown. In addition, the steps 36 and38 will, in some embodiments, be implemented as an essentiallycontinuous process of comparing sensor inputs to preset limits, andchanging the operating mode of the system by performing either step 40or 42 as indicated when a sensor input signal reaches one of the limits.It may also be noted that many different ways of implementing the twooperating modes of steps 40 and 42 may be implemented. For example, step40 may define an “off” mode for some HVAC components, while step 42defines an mode for those components. In this case,education of each onor off period may vary depending on the values of the physical andenvironmental parameters sensed at step 36. Alternatively, step 40 coulddefine a mode of operation where the HVAC system enters an on/off cycledmode at a particular duty cycle. In this case, the duty cycle may varydepending on the values of the physical and environmental parameterssensed at step 36. In both of these embodiments, the mode of operationentered into following step 40 is an energy consumption reduction mode.In some embodiments, this mode also comprises an energy recovery modewhich increases the overall efficiency of operation of the HVAC system.

FIG. 3 illustrates one specific embodiment of an apparatus constructedaccording to some aspects of the present invention. In FIG. 3, an HVACsystem 48 is shown which comprises several components. A supply air fan50 (sometimes called an “indoor fan” even though it may actually bemounted outdoors) ventilates a controlled space by forcing air throughan air supply duct 52 into a controlled space. The supply air fan 50also forces ambient air from the controlled space back into the HVACunit 48 via a return duct 54. In a chamber 56 in the HVAC system 48,this return ambient air may be heated or cooled before it is returned tothe controlled space via the air supply duct 52. To accomplish this, thechamber 56 includes air conditioner cooling coils 58 and a gas firedfurnace 60. The furnace 60 includes an electrically actuable valve 64 inits gas supply line 62. The cooling coils 58 are coupled to a compressor66 and heat exchanger coils 68 which are adjacent to a fan 70. Thesecomponents of the HVAC system may be conventional, and theirconstruction and operation will not be further described herein.Furthermore, it will be appreciated that HVAC systems come in a varietyof forms, all of which may be used with the present invention. Forexample, a given installation may use a split system with separatefurnaces and air conditioners. Systems with several heating or coolingstages may also be used. Heat pumps are another form of energy controlsystem that is compatible with the present invention.

The AC power input lines 72 are routed to the components of the HVACunit 48 through three relays 74, 76, and 78. The relay contacts of relay74 are connected between the AC power and the fan 70. The relay contactsof relay 76 are connected between the AC power and the compressor 66.The relay contacts of relay 78 are connected between the AC power andthe air supply fan 50. One side of the coils of relays 74, 76, and 78are tied to ground a their common connection to a grounded line 80. Theother side of relay coils 74 and 76 are connected to a line 82 which isrouted through a terminal block 84 and to an HVAC controller 86, whichis illustrated in more detail in FIG. 4. The electrically actuated gasvalve 64 also has one line connected to the ground line 80, and anotherline 88 which is also connected to the HVAC controller 86 through theterminal block 84. Thus, by placing and removing an appropriate voltageon lines 82 and/or 88, the HVAC controller may turn the compressor 66,fan 70, and the gas valve 64 on and off.

Although a power source for this voltage may be created in manydifferent ways known in the art, one convenient method illustrated inFIG. 3 is to use a 24 V output step down transformer 90, which has itsinput connected to the AC input lines, and its 24 V output connected tothe HVAC controller 86 and other system components which require 24 Vpower.

In addition, the coil of the third relay 78 has one side connected tothe ground line 80. The other side of the coil of the relay 78 isconnected to line 92, which is routed through the terminal block 84 andto a thermostat 94. The thermostat 94 is typically mounted in thecontrolled space. Thus, a thermostat output signal may directly controlthe operation of the supply air fan 50, without modification orinterruption by the controller 86. In typical systems, the thermostatwill activate the indoor fan 50 continuously for the entire duration ofany call for heating or cooling.

As will also be described in more detail with reference to FIG. 4, theHVAC controller receives signals from the thermostat 94 or other remotedevice on lines 96 and 98. Through these signal lines 96, 98, the HVACcontroller receives calls for heating and cooling from the controlledspace. Other information may also be transferred through, as oneexample, additional lines 97, 99. These two lines may be provided toindicate whether or not second stage heating or cooling has beenactivated by the system. Additional signals representative of othersystem parameters may also be provided. The response of the system tothese signals is described in relation to specific embodiments of thepresent invention in more detail below with reference to FIGS. 5 through8.

The HVAC controller also receives signal inputs from sensors. One sensor100 may be located in the air supply duct 52. In some embodiments, thissensor 100 has an output signal representative of the approximatetemperature of the air being supplied to the controlled space by theHVAC system. This output signal is routed to the HVAC controller vialine 104. The output signal may advantageously comprise a two-wirethermistor or thermocouple signal.

A second sensor 102 may be located in the return duct 54. This sensor102 preferably has an output signal representative of the approximatetemperature of the 5 ambient air returning from the controlled space.Also, the sensor 102 may sense the approximate humidity of the ambientair returning from the controlled space, and have a second output signalrepresentative of this parameter. This output signal, which mayadvantageously comprise a four wire interface to the HVAC controller,two for the thermistor or thermocouple and two for the humidity sensor,is routed to the HVAC controller on line 106. In addition, the sensor102 may comprise a carbon monoxide sensor. In this case, the HVACcontroller can be made to signal an audible alarm and/or shut off gasflow to the furnace 60 if excessive carbon monoxide levels are sensed.

It will be appreciated that the sensors 100 and 102 may be located inlocations different from that shown and still perform the functionrequired. As a specific example, the sensor 102 could be located in thecontrolled space itself to measure the ambient temperature and humidity.The temperature in the return air duct is simply a convenient substituteor proxy for this usually more remote location.

FIG. 4 provides a more detailed illustration of the HVAC controller 86shown in FIG. 3. In this embodiment, the HVAC controller comprises amicroprocessor 120. The term “microprocessor” in this application isintended to include any of a variety of digital processorconfigurations, including the commercially available microprocessorssuch as the X86 family from Intel. In many preferable embodiments, themicroprocessor is a commercially available microcontroller or digitalsignal processor available, for example, from Motorola or TexasInstruments.

The microprocessor 120 is coupled to the sensor inputs 104 and 106through analog signal conditioning and optical isolation circuitry 122and an analog to digital converter 124 to provide digitized datarepresentative of the environmental conditions sensed by the sensors100, 102. The microprocessor 120 is also coupled to inputs 96, 97, 98,99 from a thermostat in the controlled space or another remote device.In some advantageous embodiments, these signals comprise two levelinputs, i.e. ground and a nominal voltage, typically 24 VAC, or perhaps5 Vdc for a digital system. For example, a call for cooling will beindicated by line 96 being asserted by being pulled to the nominalvoltage. A call for heating will be indicated by line 98 being assertedby being pulled to the nominal voltage. Analogously, assertion of line97 may indicate that secondary cooling has been activated, and assertionof line 99 may indicate that secondary heating has been activated. Thesesignals are coupled to the microprocessor 120 through signalconditioning circuitry 126. In some embodiments, these signals may beoperable to interrupt microprocessor operation. In this case, wheneverlines 96 or 98, for example, are unasserted, the processor senses thatno call for heating or cooling is being made, and therefore halts anyongoing control operation and waits for the next call to re-initiatecontrol over the HVAC system components.

The microprocessor 120 is also coupled to a memory 128. This memory maystore previously received digital data obtained from the inputs 96, 97,98, 99, 104, and 106, the time at which such data was received, thelength of time the compressor or furnace has been on or off, and otherinformation relevant to HVAC operation. Some of this information may beproduced at least in part by a timer 121 implemented within themicroprocessor or as a discrete clock device. In many advantageousembodiments, the timer 121 will not generate an absolute real time, butwill be configured to measure time spans relative to some prior eventsuch as the initiation of a call for heating or cooling. Also stored inmemory 128 are predetermined setpoints against which such data iscompared to make decisions regarding HVAC operation. It can be thereforeappreciated that the memory 128 advantageously may include anon-volatile portion such as EEPROM memory as well as RAM memory. EEPROMmay be advantageous in that no backup battery is required.

The microprocessor further interfaces with an I/O port 130 forcommunicating information about the environmental and physicalparameters being monitored, and the status of the HVAC system. Thisinformation is valuable to system administrators in evaluating systemperformance and in troubleshooting system malfunctions. In addition, themicroprocessor can be reprogrammed by altering stored setpoints via theI/O port 130. The I/O port 130 may advantageously comprise an RS232serial port well known to those in the art to make communication withwidely available personal computers and handheld palmtop computersconvenient. If desired, the I/O port may comprise a wirelesstransceiver, and/or may interface to a modem for system monitoring andcontrol via RF and/or telephone communication links.

The microprocessor 120 may additionally include two outputs 132, 134which, after some buffering, drive the coils of two normally closedoutput relays 136, and 138 respectively. The contact of one of theserelays 136 is configured to output the thermostat heating call line 98to the output line 88 (see FIG. 3) which controls the gas valve 64. Thecontact of the other relay 138 is configured to output the thermostatcooling call line to the output line 82 (see FIG. 3) which controls thecompressor 66 and fan 70. Thus, the operation of the gas valve 64, thecompressor 66, and the fan 70 may be controlled by the microprocessor120 by selectively opening the contacts of the relays 136, 138. In theembodiment of FIG. 4, the normally closed relays 136 and 138 ensurenormal operation of the HVAC system if the controller is powered down oris otherwise non-operational. In this case, the heating and coolingcalls pass through the relays 136, 138, and actuate the compressor, fan,and furnace as in a conventional HVAC system.

Although more detail is provided below with regard to specificimplementations of controller operation, certain fundamental propertiescan be appreciated from examination of FIGS. 3 and 4. For instance, themicroprocessor 120 may be configured to take digital data representativeof environmental and physical conditions, make operational decisionsbased on those conditions, and dynamically control the on/off state ofenergy consuming components such as the compressor 66 and furnace 60based on the operational decisions made. The digitally based decisionmaking allows for a wide variety of sensor inputs on which to baseoperational decisions. The HVAC controller can also be convenientlyprogrammed via the I/O port for easy customization to different systemsand alterations to existing installations.

It can be appreciated that many alternative methods of system controlcan be used to improve HVAC efficiency by monitoring physical andenvironmental parameters of the system and the associated controlledspace. In FIGS. 5 through 8, specific implementations of dynamicallycontrolled energy recovery during heating and cooling calls areillustrated. The implementations shown may be advantageously producedwith appropriate configuration, via programming, of the microprocessorof FIG. 4. In the discussion below with reference to these Figures,several specific time periods, setpoints, and other parameters aredescribed. Although the specific parameters mentioned have been foundsuitable, it will be appreciated that a wide variety of options forthese parameters are possible within the scope of the present invention.Furthermore, for clarity of explanation, some of the steps set forthbelow are described terms of operations of the apparatus of FIGS. 3 and4. This apparatus is advantageous in implementing the described controlprocedures, but it will be appreciated that many different types ofphysical hardware may be used to perform the functions described.

One implementation of actions during a call for heating are illustratedin FIG. 5, beginning at step 150 of FIG. 5A, where the controllerdetermines whether a heating call is being made. If not, the systemwaits for a call at step 152. Once a call for heating has been made, thecontroller then initiates a heating call timer at step 154 to keep trackof how long this particular call for heating has been pending. Thismeasurement may be used later in the HVAC control process. Also, at step156, the system sets a minimum run timer to four minutes and starts theminimum run timer at step 158. Heating is initiated at step 160 byasserting line 88 of FIGS. 3 and 4 to open the gas valve 64. It will beappreciated that step 160 is performed immediately upon receipt of thecall for heating when apparatus in accordance with FIG. 4 is utilized asan HVAC controller. This is because the normally closed relay 138 sendsthe call to the gas valve when it is received.

Once heated air begins flowing to the controlled space, the controllermonitors the approximate air supply temperature T_(s), and compares itto a predetermined maximum setpoint, which will typically be in therange of 120 to 140 degrees Fahrenheit. As it generally takes some timefor the supply air temperature to reach this value, this comparisoninitially results, at step 162, in a decision that the supply airtemperature is less than the setpoint. In this case, at step 164, thecontroller then compares the rate of change of T_(s) with its mostrecent past value. If the rate of change of T_(s) has increased, thecontroller takes no action, waiting for 15 seconds at step 166, andloops back up to step 162 to again compare the supply air temperaturewith the predetermined maximum setpoint. If the rate of change of T_(s)is not increasing, at step 168 of FIG. 5B, the controller compares therate of change of T_(s) with another predetermined setpoint, which maybe set at approximately 0.5 to 5 degrees Fahrenheit per minute. If therate of change of T_(s) is more than this setpoint, the controller againperforms no action and waits 15 seconds at step 166. If T_(s) is greaterthan its setpoint, or the rate of change of T_(s) is less than itssetpoint, at step 170 the controller checks if the minimum run timerstarted at step 158 has timed out. If not, the controller again waits 15seconds at step 166. Thus, after initiating heating, the controllerperforms no action until the minimum run timer has timed out, and eitherT_(s) above its setpoint, or the rate of change of T_(s) is below itssetpoint. The minimum run timer thus ensures that the heating continuesin an on state for at least an amount of time which is consistent withthe manufacturers' specifications.

Once these conditions are met, the controller determines at step 172whether or not it is receiving a signal indicating that secondaryheating is also being utilized in a two stage HVAC system. Thisinformation may be received on line 99 of FIG. 4 for example. Ifsecondary heating has been activated, it indicates that no reduction inenergy transfer for the first stage coupled to the controller shouldtake place. The controller will therefore, if second stage heating isrequired to satisfy the call, loop back to continue monitoring T_(s) andits rate of change.

At step 174, the controller checks to see if the call is still pending.If not, the heat transferred has satisfied the call, and heating shouldbe discontinued. In control systems implemented with apparatusconstructed as shown in FIG. 4, it can be seen that as soon as the callfrom the thermostat is satisfied, operation of the furnace will stop,because the call signal on line 98 will no longer be present to berouted to the gas valve through the associated relay 138. As alsodescribed above with respect to the apparatus of FIG. 4, the step ofchecking for pending calls may be implemented by interrupting processoroperation when deassertion of, for example, line 98 is sensed by themicroprocessor.

If, however, at step 174 it is determined that the call is stillpending, the controller evaluates the amount of time the call has beenpending. Referring now to FIG. 5C, if at step 182 it is determined thatthe call has been pending for more than 15 minutes, at step 184 theminimum run timer is reset to eight minutes. If, at step 186, it isdetermined that the call has been pending for more than 30 minutes, atstep 188 the minimum run timer is reset to twelve minutes. If, at step190, it is determined that the call has been pending for more than 60minutes, at step 192 the controller will loop back to step 162 tocontinue monitoring T_(s) and its rate of change, thereby avoidingentering a mode of reduced energy consumption.

As mentioned above, the minimum run timer is initially set to fourminutes, so in the beginning, the pending call time will likely notsatisfy the 15, 30, and 60 minute tests defined in steps 182, 186, and190, unless other requirements such as are imposed on the supply airtemperature were not met in a short time after cooling began. Thecontroller will therefore likely not initially reset the minimum runtimer, and at step 192, checks the ambient air temperature of the spaceby looking at T_(r), the temperature of the air in the return duct. Ifthis temperature is lower than 55 degrees F, the controller again loopsback to step 162 to continue monitoring T_(s) and its rate of change.However, if T_(r) is greater than 55 degrees F, the controller willinitiate energy recovery mode at step 194. This step of comparing T_(r)with a fixed value allows the system to inhibit energy recovery when thethermostat setpoint is far different from the actual temperature of thecontrolled space. The HVAC unit will thus operate at maximum outputuntil the temperature of the controlled space reaches a more comfortablevalue.

Referring back to FIGS. 3 and 4, in this embodiment the entering ofenergy recovery mode may involve simply the opening of the valve 64 byopening the relay 98 to remove the call signal from line 88. Thisreduces the energy consumption of the HVAC unit dramatically. However,the supply air fan remains operational, so that air can continue to flowthrough the system, drawing heat from system components that wouldotherwise be radiated or conducted away and lost. As shown by FIG. 5,this energy recovery step 194 is taken if (1) the minimum run timer issatisfied, (2) either Ts is greater than its maximum setpoint or therate of change of T_(s) is less than its minimum setpoint, and (3) thetemperature of the controlled space is greater than 55 degrees F.Otherwise, the heating initiated at step 160 is continued.

Following the initiation of recovery at step 194, the short cycle timeris started at step 196. Once recovery is initiated at step 194 and thefurnace is off, the air in the supply duct begins to cool off, and todraw heat from the ducting material, other mechanical components of theHVAC system, and structural components of the controlled space. Thisenergy recovery continues as the supply air temperature drops toward theambient temperature, and the controller will wait until certainconditions are met before re-initiating heating.

Therefore, at step 198 of FIG. 5D, the controller then determineswhether or not the supply air temperature is above a minimum setpoint,typically set at 90 to 100 degrees F. If it is, the controller thenchecks, at step 200, if the rate of change of the temperature of thecontrolled space is positive, that is, is the controlled space stillgetting warmer. If it is, the controller moves to step 202, anddetermines if the rate of change (in the negative direction this time)of the supply air temperature has increased over that previouslyrecorded. If it has, the controller takes no action, and waits 15seconds at step 204 before looping back to step 198 and re-checking thesupply air temperature.

If the rate of change is not decreasing, the controller then checks atstep 206 if the rate of change of the supply air temperature is greaterthan a predetermined setpoint, which may advantageously be set at 0.5 to5 degrees F per minute. If it is, the controller again takes no action,and waits 15 seconds at step 204 before looping back to step 198 andrechecking the supply air temperature.

If any one of these three conditions hold: (1) at step 198 T_(s) is lessthan the minimum setpoint, (2) at step 200 the controlled space is nolonger increasing in temperature, or (3) the rate of change of T_(s) isless than a predetermined setpoint, then the controller will move out ofthis 15 second increment waiting loop and first check at step 208 to seeif the call for heating is still pending. If the heating call has beensatisfied (i.e., the call is no longer pending), the system loops up tostate 150, and waits for the next call. If the heating call has not beensatisfied, heating should be re-initiated. In this case, the short cycletimer, which was started at step 196, is checked at step 210 to see ifit is timed out. If not, the controller then waits, in five secondincrements illustrated by step 212, for the short cycle timer to timeout. When it has been determined that the short cycle timer timed out atstep 210, the controller loops back to step 160 and reinitiates aheating cycle by turning the gas valve for the furnace back on, by, forexample, allowing the relay 138 to close, and outputting the call signalon line 88 again.

FIG. 6 illustrates a specific implementation of energy recovery during acooling call according to aspects of the present invention. Once again,the implementation shown is advantageously produced with appropriateconfiguration, via programming, of the microprocessor of FIG. 4. Manyparallels to the heating flowchart of FIG. 5 will be apparent, althoughsome differences exist.

Actions during a call for cooling are illustrated in FIG. 6, beginningat step 220 of FIG. 6A, where the controller determines whether acooling call is being made. If not, the system waits at step 222. Once acall for cooling has been made, the controller then initiates a coolingcall timer at step 224 to keep track of how long this particular callfor cooling has been pending. This measurement may be used later in theHVAC control process. Also, at step 226, the system sets a minimum runtimer to four minutes and starts the minimum run timer at step 228.Cooling is initiated at step 230 by asserting line 82 of FIGS. 3 and 4to turn on the compressor 66 and fan 70. It will be appreciated thatstep 230 is performed immediately upon receipt of the call for coolingwhen apparatus in accordance with FIG. 4 is utilized as an HVACcontroller. This is because the normally closed relay 136 sends the callto the compressor and outdoor fan when it is received.

Once cooled air begins flowing to the controlled space, the controllermonitors the approximate air supply temperature T_(s), and compares itto a predetermined minimum setpoint, which will typically be in the 50to 60 degrees Fahrenheit range. As it generally takes some time for thesupply air temperature to reach this value, this comparison initiallyresults, at step 232, in a decision that the supply air temperature isgreater than the setpoint. In this case, at step 234, the controllerthen compares the rate of change of T_(s) with its most recent pastvalue. If the rate of change of T_(s) increased, the controller takes noaction, waiting for 15 seconds at step 236, and loops back up to step232 to compare the supply air temperature with the predetermined minimumsetpoint. If the rate of change of T_(s) is not increasing, at step 238of FIG. 6B the controller compares the rate of change of T_(s) withanother predetermined setpoint, typically set at 0.5 to 5 degreesFahrenheit per minute. If the rate of change of T_(s) is more than thissetpoint, the controller again performs no action and waits 15 secondsat step 236.

If either T_(s) is greater than its setpoint, or the rate of change ofT_(s) is less than its setpoint, at step 240 the approximate humidity ofthe controlled space may be checked. If this humidity is greater than amaximum setpoint, the controller again waits 15 second and loops back tostep 232. Cooling will thus continue at maximum output during highhumidity periods. If the humidity is lower than the setpoint, thecontroller determines at step 242 whether or not it is receiving asignal indicating that secondary cooling is also being utilized in a twostage HVAC system. This information may be received on line 97 of FIG. 4for example. If secondary cooling has been activated, it indicates thatno reduction in energy transfer for the first stage coupled to thecontroller should take place. The controller will therefore, if secondstage cooling is required to satisfy the call, loop back to continuemonitoring T_(s) and its rate of change.

If second stage cooling is not activated, the status of the call forcooling is checked at step 244. If the heat transferred has satisfiedthe call, cooling should be discontinued. In control systems implementedwith apparatus constructed as shown in FIG. 4, it can be seen that assoon as the call from the thermostat is satisfied, operation of thecompressor and fan will stop, because the call signal on line 96 will nolonger be present to be routed to the compressor 66 and fan 70 throughthe associated relay 136. The system thus loops back to step 220 andawaits the next call for cooling. As also described above with respectto the apparatus of FIG. 4, the step of checking for pending calls maybe implemented by interrupting processor operation when deassertion of,for example, line 96 is sensed by the microprocessor.

Moving back to step 244, if the cooling call has not been satisfied, thecontroller waits for the minimum run timer to time out by checking itsstatus at step 256, and waiting in five second increments at step 258until the minimum run timer has timed out. The minimum run timer ensuresthat the compressor operates for a time at least as long as suggested bythe compressor manufacturer before entering a recovery cycle.

Once the minimum run timer has expired, the controller moves to step 260of FIG. 6C and evaluates the amount of time the call has been pending.If, at step 260, it is determined that the call has been pending formore than 15 minutes, at step 262 the minimum run timer is reset toeight minutes. If, at step 264, it is determined that the call has beenpending for more than 30 minutes, at step 266 the minimum run timer isreset to twelve minutes. If, at step 268, it is determined that the callhas been pending for more than 60 minutes, the controller will loop backto step 232 to continue monitoring T_(s) and its rate of change,bypassing entry into an energy recovery mode.

As mentioned above, the minimum run timer is initially set to fourminutes, so in the beginning, the pending call time will likely notsatisfy the 15, 30, and 60 minute tests defined in steps 260, 264, and268, unless other requirements such as are imposed on the supply airtemperature were not met in a short time after cooling began. Thecontroller will therefore likely not initially reset the minimum runtimer, and at step 270, checks the ambient air temperature of thecontrolled space by looking at T_(r), the temperature of the air in thereturn duct. If this temperature is higher than 80 degrees F, thecontroller again loops back to step 232 to continue monitoring T_(s) andits rate of change. However, if T_(r) is less than 80 degrees F, thecontroller will initiate energy recovery mode at step 272. In this caseas well, recovery mode is not entered if the temperature of thecontrolled space is uncomfortable.

Referring back to FIGS. 3 and 4, in this embodiment energy recover modemay involve simply the shutting down of the compressor 66 and fan 70 byremoving the cooling call signal from line 82 by opening the associatedrelay 136. This reduces the energy consumption of the HVAC unitdramatically. However, the supply air fan remains operational, so thatair can continue to flow through the system, losing heat to systemcomponents that would otherwise remain in the controlled space which isbeing cooled. As shown by FIG. 5, this step 272 is taken if (1) theminimum run timer is satisfied, (2) either T_(s) is less than itsminimum setpoint or the rate of change of T_(s) is less than its minimumsetpoint, and (3) the temperature of the controlled space is less than80 degrees F. Otherwise cooling initiated at step 230 is continued.

Following the initiation of recovery at step 272, the short cycle timeris started at step 274. Once recovery is initiated and the compressor 66is off, the air in the supply duct begins to warm, and to lose heat tothe cold ducting material, other mechanical components of the HVACsystem, and structural elements of the controlled space. Energy recoverythus continues as the supply air temperature warms toward the ambienttemperature, and the controller will wait until certain conditions aremet before re-initiating cooling.

Referring now to FIG. 6D, at step 276 the controller then determineswhether or not the supply air temperature is below a maximum setpoint,typically set at 60 to 70 degrees F. If it is, the controller thenchecks, at step 278, if the rate of change of the temperature of thecontrolled space is negative, that is, is the controlled space stillgetting cooler. If it is, the controller moves to step 280, anddetermines if the rate of change (in the positive direction this time)of the supply air temperature has increased over that previouslyrecorded. If it has, the controller takes no action, and waits 15seconds at step 282 before looping back to step 276 and re-checking thesupply air temperature.

If the rate of change is not decreasing, the controller then checks atstep 284 if the rate of change of the supply air temperature is greaterthan a predetermined setpoint, which may be set at 0.5 to 5 degrees perminute. If it is, the controller again takes no action, and waits 15seconds at step 282 before looping back to block 276 and re-checking thesupply air temperature.

If any one of these three conditions hold: (1) at step 276 T_(s) is morethan the maximum setpoint, (2) at step 278 the controlled space is nolonger decreasing in temperature, or (3) the rate of change of T_(s) isless than a predetermined setpoint, then the controller will move out ofthis 15 second increment waiting loop and first check at step 286 to seeif the call for cooling is still pending. If the cooling call has beensatisfied (i.e., the call is no longer pending), the controller loopsback to block 220 to wait for the next call for cooling. If the coolingcall has not been satisfied, cooling should be re-initiated. In thiscase, the short cycle timer, which was started at step 274, is checkedat step 288 to see if it is timed out. If not, the controller thenwaits, in five second increments illustrated by step 290, for the shortcycle timer to time out. When it has been determined that the shortcycle timer timed out at step 288, the controller loops back to step 230and reinitiates a cooling cycle by turning the air conditioningcompressor back on, by, for example, allowing the relay 138 to close,and outputting the call signal on line 88 again. The short cycle timertherefore prevents the restart of the compressor for a period at leastas long as that recommended by the compressor manufacturer.

Another alternative control procedure for cooling is illustrated inFIGS. 7A through 7C. As with the procedures of FIGS. 5 and 6, the systembegins in FIG. 7A at step 300 monitoring whether or not a call forcooling is being made. If not, the system waits for a call at step 302.If a call for cooling is has been received, the system moves to block304, and checks the approximate humidity of the controlled space or, asexplained above, the approximate humidity in the return air duct. If themeasured humidity is greater than 60%, the controller is inhibited frominitiating energy recovery, and conventional cooling control isperformed at step 306. Typically, in the conventional control mode ofblock 306, the system simply cools at maximum capacity for the durationof any call for cooling, and is shut off otherwise. As one example, ifthe apparatus of FIGS. 3 and 4 is used to implement this method, theprocessor 120 is inhibited from opening either relay 136, 138.

If the humidity is below 60%, a cooling minimum run timer is set andstarted. If the call has just been received, and no prior energyrecovery cycles have taken place, at step 308 a minimum run timer forthe first cooling cycle is set and started. As will be discussed furtherbelow, if the system is returning from a recovery cycle, a minimum runtimer of possibly different duration is set and Started at step 310.Although suitable systems may be created using a run timer of the sameduration for all cycles, it may be desirable for compressor operation ifthe first cooling cycle, which may follow lengthy off period, issomewhat longer than the cooling periods between recovery cycles. Theminimum run timer of block 308 thus only affects compressor operationduring the first cycle after receiving a call for cooling.

Moving now to block 312, the controller computes a target temperaturefor the air returning from the controlled space. This temperature iscomputed to ensure that the controlled space temperature is reduced by aminimum amount prior to the initiation of energy recovery. In someembodiments of the present invention, the target temperature may becalculated with the current return air temperature, a user programmableminimum cooling rate (which may advantageously be set to 2 to 5 degreesF per hour) and the initial set value of the minimum run timer, whichmay advantageously also be user programmable. In one embodiment, thetarget temperature is calculated by calculating the temperaturereduction produced by a cooling of the air produced by maintaining theuser programmed minimum rate for the duration of the minimum run timerinitial setting. For example, if the programmable minimum cooling rateis 3 degrees per hour, and the minimum run timer is set to 6 minutes,the target temperature is set to 3 degrees per hour times 0.1 hours, or0.3 degrees cooler than the current return air temperature.

At step 314 cooling is initiated. As discussed above with reference toFIGS. 5 and 6, this will occur immediately upon receipt of the call ifthe apparatus implementing this procedure is made in accordance withFIG. 4.

While the air conditioner is activated, at block 316 the system measuresthe temperature of the air in the controlled space. As mentioned above,this measurement can be made by directly sensing temperature in thecontrolled space, or by sensing the temperature in a return air duct. Ifthe return temperature has not cooled to the target temperature, thecontroller performs no further action and at block 318 waits fiveseconds before making another measurement at block 316. Thus, thecontroller remains in the loop defined by blocks 316 and 318 until theapproximate temperature of the air in the controlled space drops belowthe target temperature.

Referring now to FIG. 7B, once the approximate temperature of thecontrolled space drops below the target temperature, the system checksif the cooling call is satisfied at step 320. If it is, the systemreturns to block 300 on FIG. 7A, and waits for the next cooling call. Ifthe cooling call has not been satisfied, at steps 322 and 324 thecontroller checks the status of the minimum run timer which was set andstarted at step 308 (or step 310 if this is not the first cyclefollowing a call for cooling) described above. As long as this timer hasnot expired, the system waits in five second increments at step 326until it has. If the system is a two-stage type, at block 324 thecontroller also monitors whether or not the second stage is currentlyactivated. If the second stage is activated, a recovery cycle will beinappropriate, and the system will again wait in five second incrementsrepresented by block 326 until the second stage is off. It can beappreciated that all of these steps may be essentially continuouslyperformed, with the microprocessor continuously monitoring the status ofthe pending call, temperatures, and timer, and waiting until allrequired conditions are fulfilled before moving to the next step.

Once the timer has expired, the controller initiates energy recovery atstep 328, by, for example, opening relay 138 if the apparatus of FIG. 4is used to implement this control procedure. Following the initiation ofrecovery at step 328, the short cycle timer is set and started at step330. Once recovery is initiated and the compressor 66 is off, the air inthe supply duct begins to warm, and to lose heat to the cold ductingmaterial, other mechanical components of the HVAC system, and structuralelements of the controlled space. Energy recovery thus continues as thesupply air temperature warms, and the controller will wait until certainconditions are met before re-initiating cooling.

At this stage of the procedure, the short cycle timer, which was startedat step 330, is checked at step 332 to see if it is timed out. If not,the controller then waits, in five second increments illustrated by step334, for the short cycle timer to time out. The short cycle timer thusensures an off time which may advantageously be programmed to helpensure that compressor operation is within the manufacturer'sspecifications.

As shown on FIG. 7C, once the short cycle timer times out, at step 336the system check to see if the approximate temperature of the supply airis within 0.5 degrees of the approximate temperature of the controlledspace (as may be determined by monitoring the air temperature in thereturn duct). If it is, this indicates that supply and returntemperatures are equilibrating, and that therefore significant energyrecovery from system components is no longer occurring. As illustratedby step 338, the system monitors the call status to see if the coolingcall has been satisfied. If the cooling call has been satisfied, thesystem moves back to the start of the procedure at block 300 of FIG. 7A,and waits for the next call for cooling.

If the cooling call has not been satisfied, the system will then loopback to block 310 to begin the next on-cycle of the air conditioner. Asbefore, after the system repeats the initiation of cooling at block 310,the minimum run timer is again set and started, and a new targettemperature is calculated using the current air temperature of thecontrolled space as a new base point.

Returning now to step 336 of FIG. 7C, if the supply temperature T_(s) isnot within 0.5 degrees of the return temperature T_(r), the systemchecks, at block 340, whether or not the current rate of change of thesupply temperature is less than 10% of the maximum rate of changedetected during the present off-cycle. In other words, is the slope ofthe supply temperature vs. time flattening out significantly, therebyindicating the onset of equilibration and reduction in the rate ofenergy recovery from system components. If the rate of change of thesupply temperature is still greater than 10% of the maximum obtainedduring the present off cycle, the system waits for 5 seconds at block342 and loops back to block 336 to re-compare the temperature of thereturn and supply air temperatures. Once this rate of change conditionis satisfied, the controller loops back to block 310 to reinitiatecooling assuming the call for cooling is still pending. However, if thepending call gets satisfied at some point during the recovery cycle, thecontroller loops back to block 300 to await the next call.

A system operating in accordance with the procedure of FIGS. 7A through7C will therefore continue to operate an air conditioning unit until acertain target temperature is reached and at least one minimum run timerhas expired. Energy recovery is then initiated, which continues untilthe supply and return air temperatures are close to one another, oruntil the rate of change of the supply air temperature flattensconsiderably. The system cycles between the on-state and the recoverystate until the call for cooling is satisfied.

Another alternative control procedure for heating is illustrated inFIGS. 8A through 8C. This scheme is similar to that described withreference to FIGS. 7A through 7C. As with the procedure of FIG. 7, thesystem begins in FIG. 8A at step 350 monitoring whether or not a callfor heating is being made. If not, the system waits for a call at step352.

Once a call has been made, a heating minimum run timer is set andstarted. First, at step 358, a minimum run timer for the first heatingcycle is set and started. As in the FIG. 7 embodiment described above, aminimum run timer of a different duration may be set and started at step360 when the system loops back from a recovery cycle. Also in analogywith the FIG. 7 embodiment above, the first heating minimum run timermay be set longer than the minimum run timer which is effective forsubsequent furnace cycles.

At step 362, the controller computes a target temperature for the airreturning from the controlled space. This temperature is computed toensure that the controlled space temperature is increased by a minimumamount prior to the initiation of energy recovery. In some embodimentsof the present invention, the target temperature may be calculated withthe current return air temperature, a user programmable minimum heatingrate (which may advantageously be set to 2 to 5 degrees F per hour) andthe initial set value of the minimum run timer, which may advantageouslyalso be user programmable. In one embodiment, the target temperature iscalculated by calculating the temperature increase produced by a heatingof the air produced by maintaining the user programmed minimum rate forthe duration of the minimum run timer initial setting. For example, ifthe programmable minimum heating rate is 3 degrees per hour, and theminimum run timer is set to 6 minutes, the target temperature is set to3 degrees per hour times 0.1 hours, or 0.3 degrees warmer than thecurrent return air temperature.

At step 364, heating is initiated. As discussed above with reference toFIGS. 5 and 6, this will occur immediately upon receipt of the call ifthe apparatus implementing this procedure is made in accordance withFIG. 4.

While the furnace is activated, at block 366 the system measures thetemperature of the air in the controlled space. As mentioned above, thismeasurement can be made by directly sensing temperature in thecontrolled space, or by sensing the temperature in a return air duct. Ifthe return temperature has not warmed to the target temperature, thecontroller performs no further action and at block 368 waits fiveseconds before making another measurement at block 366. Thus, thecontroller remains in the loop defined by blocks 366 and 368 until theapproximate temperature of the air in the controlled space increasesabove the target temperature.

Referring now to FIG. 8B, once the approximate temperature of thecontrolled space rises above the target temperature, the system checksif the heating call is satisfied at step 370. If it is, the systemreturns to block 350 on FIG. 8A, and waits for the next heating call. Ifthe heating call has not been satisfied, at step 372 the controllerchecks the status of the minimum run timer which was set and started atstep 358 (or step 360 if this is not the first cycle following a callfor heating) described above. As long as this timer has not expired, thesystem waits in five second increments at step 376 until it has. If thesystem is a two-stage type, at block 374 the controller also monitorswhether or not the second stage is currently activated. If the secondstage is activated, a recovery cycle will be inappropriate, and thesystem will again wait in five second increments represented by block376 until the second stage is off. As discussed above, it can beappreciated that all of these steps may be essentially continuouslyperformed, with the microprocessor continuously monitoring the status ofthe pending call, temperatures, and timer, and waiting until allrequired conditions are fulfilled before moving to the next step.

Once the timer has expired, the controller initiates energy recovery atstep 378, by, for example, opening the relay contacts of the relay 136if the apparatus of FIG. 4 is used to implement this control procedure.Following the initiation of recovery at step 378, the short cycle timeris set and started at step 380. Once recovery is initiated and thefurnace is off, the air in the supply duct begins to cool, and to removeheat from the warm ducting material, other mechanical components of theHVAC system, and structural elements of the controlled space. Energyrecovery thus continues as the supply air temperature cools toward theambient temperature, and the controller will wait until certainconditions are met before re-initiating heating.

At this stage of the procedure, the short cycle timer, which was startedat step 380, is checked at step 382 to see if it is timed out. If not,the controller then waits, in five second increments illustrated by step384, for the short cycle timer to time out. The short cycle timer thusensures an off time which may advantageously be programmed to helpensure that furnace operation is within the manufacturer'sspecifications.

Referring now to FIG. 8C, once the short cycle timer times out, at step386 the system check to see if the approximate temperature of the supplyair is within 0.5 degrees of the approximate temperature of thecontrolled space (as may be determined by monitoring the air temperaturein the return duct). If it is, this indicates that supply and returntemperatures are equilibrating, and that therefore significant energyrecovery from system components is no longer occurring. As illustratedby step 388, the system monitors the call status to see if the heatingcall has been satisfied at block 388. If the heating call has beensatisfied, the system moves back to the start of the procedure at block350 of FIG. 8A, and waits for the next call for heating.

If the heating call has not been satisfied, the system will then loopback to block 358 to begin the next on-cycle of the furnace. As before,after the system repeats the initiation of heating at block 360, theminimum run timer is again set and started, and a target temperature iscalculated using the current air temperature of the controlled space asa new base point. It may also be noted that for this and subsequenton-cycles, the minimum first run timer is not re-set or re-started.Thus, the system effectively waits only for the attainment of the targettemperature and the expiration of the minimum run timer at block 374before initiating another energy recovery cycle at block 378 of FIG. 8B.

Returning now to step 386 of FIG. 8C, if the supply temperature T_(s) isnot within 0.5 degrees of the return temperature T_(r), the systemchecks, at block 390, whether or not the current rate of change of thesupply temperature is less than 10% of the maximum rate of changedetected during the present off-cycle. In other words, is the slope ofthe supply temperature vs. time flattening out significantly, therebyindicating the onset of equilibration and reduction in the rate ofenergy recovery from system components. If the rate of change of thesupply temperature is still greater than 10% of the maximum obtainedduring the present off cycle, the system waits for 5 seconds at block392 and loops back to block 386 to re-compare the temperature of thereturn and supply air temperatures. Once this rate of change conditionis satisfied, the controller loops back to block 360 to reinitiateheating assuming the call for heating is still pending. However, if thepending call gets satisfied at some point during the recovery cycle, thecontroller loops back to block 350 to await the next call.

Thus, in analogy with FIGS. 7A through 7C, a system operating inaccordance with the procedure of FIGS. 8A through 8C will continue tooperate a furnace until a certain target temperature is reached and atleast one minimum run timer has expired. Energy recovery is theninitiated, which continues until the supply and return air temperaturesare close to one another, or until the rate of change of the supply airtemperature flattens considerably. The system cycles between theon-state and the recovery state until the call for heating is satisfied.

It can be appreciated that in many embodiments of the above describedprocedures, the various evaluation of environmental and physicalparameters such as temperature, humidity, durations, etc. will beessentially constantly performed. For example, when implementing thesemethods with the controller of FIG. 4, the humidity can be constantlymonitored, and the controller can be disabled from affecting thenormally closed state of the relays 136, 138 until the humidity dropsbelow its setpoint. Thus, the particular order and sequence of theflowcharts of FIGS. 5 through 8 is not intended to indicate that thisorder is required. In some embodiments of these methods implemented withthe apparatus of FIGS. 3 and 4 for example, it can be seen that thecalls for cooling and heating still directly control the furnace andcompressor, and thus the controller cannot force heating or cooling tooccur when no call is present. Thus, the initiation of heating orcooling can be implemented by simply allowing the normal cooling orheating calls to pass through the controller. Many different specificimplementations of parameter monitoring and HVAC control in accordancewith principles of the present invention will be possible to those ofordinary skill in the art based on this disclosure.

The graphs of FIGS. 9 through 12 illustrate energy consumption levelsand energy output levels for a typical five ton HVAC system respondingto an example 30 minute call for environmental modification with anenergy controller operating in accordance with some principles of thepresent invention. FIG. 9 illustrates BTU per minute of gas use over the30 minute period. Drop off 400, 402, 404 in gas use indicate theinitiation of energy recovery. Increases in gas use 406, 408, indicatere-initiation of heating after each period of energy recovery. When thegas is on, an energy equivalent of 4,444 BTUs per minute are beingconsumed in the furnace. The average energy consumption over the severalon/off cycles of gas use is 2,645 BTUs.

FIG. 10 is a graph of BTU output to the controlled space during theperiod of gas use shown in FIG. 9. The instantaneous BTU output 410 hasa peak of approximately 3400 BTU per minute delivered to the controlledspace. The average BTU output 412 to the room is approximately 2455 BTUper minute. Due to energy recovery from system components, the decreasein average energy input is greater than the decrease in average energyoutput. An increase in heating efficiency is thus attained.

FIGS. 11 and 12 demonstrate a similar effect in a cooling mode. Thecompressor is effectively cycled, resulting in energy consumptionreductions at points 416 and 418 of approximately 80%. The remaining 20%of the power consumed during energy recover periods is consumed mainlyby the air supply fan, which as described above, preferably remainsoperational. Referring now to FIG. 12, it can be seen that in a manneranalogous to the heating graph of FIG. 10, the instantaneous BTU perminute removed the controlled space in cooling 420 rises and fallsdepending on whether or not the HVAC controller is in energy recoverymode. The average energy transfer 422 however, remains high enough toproduce a significant increase in cooling efficiency.

The above described invention therefore provides many advantages overprior art HVAC control systems and methods. One major benefit is theprovision of reduced energy consumption without significantly reducingthe comfort of occupants of the controlled space. In addition,safeguards such as minimum run timers and short cycle timers may beprovided to protect the HVAC equipment form over-cycling.

The digital nature of the preferred embodiment also allows for aconvenient programmable mode of operation which allows energy savings tobe determined empirically with a high degree of accuracy. The HVACcontroller of the present invention can be programmed to refrain fromentering energy recovery mode on alternating 24 hour periods. Relevantdata such as on and off times and energy outputs may be stored in theon-board memory over an extended 30 to 60 day test period. The totalenergy consumption of the environmental control system is compared forthe two periods, one with the energy recovery mode operational, and theother without. The energy consumption of the system can be estimated bymeasuring on-time durations for heating and cooling, or can be measuredmore directly by gas flow sensors and ammeters or wattmeters situated toprovide essentially direct power consumption measurements. Furthermore,variations in the duration of recovery disabled mode vs. recoveryenabled mode can be made depending on the nature of the specificinstallation. Advantageously, the data can be made available to systemadministrators via the I/O port.

The foregoing description details certain preferred embodiments of thepresent invention and describes the best mode contemplated. It will beappreciated, however, that no matter how detailed the foregoing appearsin text, the invention can be practiced in many ways. As is also statedabove, it should be noted that the use of particular terminology whendescribing certain features or aspects of the present invention shouldnot be taken to imply that the broadest reasonable meaning of suchterminology is not intended, or that the terminology is being redefinedherein to be restricted to including any specific characteristics of thefeatures or aspects of the invention with which that terminology isassociated. The scope of the present invention should therefore beconstrued in accordance with the appended Claims and any equivalentsthereof.

What is claimed is:
 1. A method of controlling an HVAC systemcomprising: receiving a call for cooling; turning on an air conditioningcompressor; monitoring the on-time of said air conditioning compressor;sensing a controlled space temperature; comparing said controlled spacetemperature with a predetermined value; and shutting off said airconditioning compressor when both (1) said on-time reaches at least apredetermined minimum time period, and (2) said controlled spacetemperature is less than said predetermined value.
 2. The method ofclaim 1, wherein said predetermined minimum substantially preventsover-cycling of said air conditioning compressor.
 3. The method of claim1, wherein said predetermined value substantially minimizes occupantdiscomfort.
 4. The method of claim 3, wherein said predetermined valueis approximately 80 degrees Fahrenheit.
 5. The method of claim 1,additionally comprising turning said air conditioning compressor back onin response to an increase in the temperature of said controlled space.